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Periods of oceanic anoxia have had a major influence on the evolutionary history of Earth and are often contemporaneous with mass extinction events. Changes in global (as opposed to local) redox conditions can be potentially evaluated using U system proxies. The intensity and timing of oceanic redox changes associated with the end-Permian extinction horizon (EH) were assessed from variations in 238 U∕ 235 U (δ 238 U) and Th/U ratios in a carbonate section at Dawen in southern China. The EH is characterized by shifts toward lower δ 238 U values (from −0.37‰ to −0.65‰), indicative of an expansion of oceanic anoxia, and higher Th/U ratios (from 0.06 to 0.42), indicative of drawdown of U concentrations in seawater. Using a mass balance model, we estimate that this isotopic shift represents a sixfold increase in the flux of U to anoxic facies, implying a corresponding increase in the extent of oceanic anoxia. The intensification of oceanic anoxia coincided with, or slightly preceded, the EH and persisted for an interval of at least 40,000 to 50,000 y following the EH. These findings challenge previous hypotheses of an extended period of whole-ocean anoxia prior to the end-Permian extinction.carbonates | uranium isotopes | paleoredox T he end-Permian extinction represents the largest mass extinction in Earth history, with the demise of an estimated 90% of all marine species (1). While it has been extensively studied, the exact nature and cause of the end-Permian extinction remains the subject of intense scientific debate. Proposed kill mechanisms have included a nearby supernova, bolide impacts, periods of extreme volcanism (e.g., Siberian Traps), extensive glaciation, and widespread oceanic anoxia (2). Evidence for shallow-ocean anoxia in conjunction with the end-Permian mass extinction is widespread (3-6), but the intensity and timing of oceanic redox changes remain uncertain (7-10). Recent hypotheses have invoked the release of hydrogen sulfide gas (H 2 S) from seawater as a kill mechanism (11-13). Such models call upon strong expansion of oceanic anoxia below the oxygenated surface layer to allow buildup of H 2 S, followed by an upward excursion of the chemocline that releases the poisonous gas into the atmosphere (13). In this study, we examine the 238 U∕ 235 U and Th/U (thorium/ uranium) ratios in a carbonate section spanning the end-Permian extinction horizon (EH) to evaluate the timing and scale of these possibilities. Samples for this study were collected from the Dawen section of the Yangtze Block in southern China (Fig. 1), which has been correlated with the global stratotype section and point (GSSP) of the Permian-Triassic boundary at Meishan (14).Due to the geochemical properties of U, the ratio of 238 U∕ 235 U can be used as a tool to investigate the history of ocean oxygenation at a global scale, as opposed to the local redox information provided by most commonly used proxies. The long residence time (∼500 ky) of U in the oceans leads to a homogeneous U concentration in seawater (15, 16), as well as to ...

Previous work has shown uranium (U) isotope fractionation between natural ferromanganese crusts and seawater. Understanding the mechanism that causes (238)U/(235)U fractionation during adsorption to ferromanganese oxides is a critical step in the utilization of (238)U/(235)U as a tracer of U adsorption reactions in groundwater as well as a potential marine paleoredox proxy. We conducted U adsorption experiments using synthetic K-birnessite and U-bearing solutions. These experiments revealed a fractionation matching that observed between seawater and natural ferromanganese sediments: adsorbed U is isotopically lighter by ∼0.2‰ (δ(238/235)U) than dissolved U. As the redox state of U does not change during adsorption, a difference in the coordination environment between dissolved and adsorbed U is likely responsible for this effect. To test this hypothesis, we analyzed U adsorbed to K-birnessite in our experimental study using extended X-ray absorption fine structure (EXAFS) spectroscopy, to obtain information about U coordination in the adsorbed complex. Comparison of our EXAFS spectra with those for aqueous U species reveals subtle, but important, differences in the U-O coordination shell between dissolved and adsorbed U. We hypothesize that these differences are responsible for the fractionation observed in our experiments as well as for some U isotope variations in nature.

The 238U/235U isotope ratio has long been considered invariant in meteoritic materials (equal to 137.88). This assumption is a cornerstone of the high-precision lead-lead dates that define the absolute age of the solar system. Calcium-aluminum-rich inclusions (CAIs) of the Allende meteorite display variable 238U/235U ratios, ranging between 137.409 +/- 0.039 and 137.885 +/- 0.009. This range implies substantial uncertainties in the ages that were previously determined by lead-lead dating of CAIs, which may be overestimated by several million years. The correlation of uranium isotope ratios with proxies for curium/uranium (that is, thorium/uranium and neodymium/uranium) provides strong evidence that the observed variations of 238U/235U in CAIs were produced by the decay of extant curium-247 to uranium-235 in the early solar system, with an initial 247Cm/235U ratio of approximately 1.1 x 10(-4) to 2.4 x 10(-4).

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Events occurring within the first 10 million years of the Solar System's approximately 4.5 billion-year history, such as formation of the first solids, accretion, and differentiation of protoplanetary bodies, have determined the evolutionary course of our Solar System and the planetary bodies within it. The application of high-resolution chronometers based on short-lived radionuclides is critical to our understanding of the temporal sequence of these critical events. However, to map the relative ages from such chronometers onto the absolute time scale, they must be "anchored" to absolute ages of appropriate meteoritic materials using the high-precision lead-lead (Pb-Pb) chronometer. Previously reported Pb-Pb dates of the basaltic angrite meteorites, some of which have been used extensively as time anchors, assumed a constant 238 U∕ 235 U ratio (¼137.88). In this work, we report measurements of 238 U∕ 235 U ratios in several angrites that are distinct from the previously assumed value, resulting in corrections to the Pb-Pb ages of ≥1 million years. There is no resolvable variation in the 238 U∕ 235 U ratio among the angrite bulk samples or mineral separates, suggesting homogeneity in the U isotopic composition of the angrite parent body. Based on these measurements, we recalculated the Pb-Pb age for the commonly used anchor, the D'Orbigny angrite, to be 4563.37 AE 0.25 Ma. An adjustment to the Pb-Pb age of a time anchor (such as D'Orbigny) requires a corresponding correction to the "model ages" of all materials dated using that anchor and a shortlived chronometer. This, in turn, has consequences for accurately defining the absolute timeline of early Solar System events.anchor | geochronology T he time from the formation of the first solids in the Solar System to the accretion and differentiation of protoplanetary embryos is less than approximately 10 million years (Ma) (ref. 1 and references therein), and these events have determined the evolutionary course of our Solar System and the planetary bodies within it. Knowledge of the precise timing of events during this period is critical to a broader understanding of how star systems and planetary bodies form and evolve. As such, precise and accurate geochronology, allowing the resolution of events occurring within this critical approximately 10 Ma interval, is required to understand this earliest sequence of events in the Solar System. A very limited number of chronometers can provide the sub-Ma precision necessary to resolve early Solar System events. These include the long-lived lead-lead (Pb-Pb) chronometer, and the short-lived chronometers such as 26 Al-26 Mg (t 1∕2 approx. 0.72 Ma), 53 Mn-53 Cr (t 1∕2 approx. 3.7 Ma), and 182 Hf-182 W (t 1∕2 approx. 9 Ma). The Pb-Pb chronometer is based on two radioactive isotopes of uranium, utilizing the distinct decay schemes of 235 U → 207 Pb (t 1∕2 approx. 704 Ma) and 238 U → 206 Pb (t 1∕2 approx. 4.47 Ga) to calculate an absolute age of the sample. The short-lived or extinct radionuclide chronometers have parent isotopes ...

A long-standing paradigm assumes that the chemical and isotopic composition of many elements in the bulk silicate Earth are the same as in chondrites1–4. However, the accessible Earth has a greater 142Nd/144Nd than chondrites. Because 142Nd is the decay product of now-extinct 146Sm (t1/2= 103 million years5), this 142Nd difference seems to require a higher-than-chondritic Sm/Nd of the accessible Earth. This must have been acquired during global silicate differentiation within the first 30 million years of Solar System formation6 and implies the formation of a complementary 142Nd-depleted reservoir that either is hidden in the deep Earth6, or was lost to space by impact erosion3,7. Whether this complementary reservoir existed, and whether or not it has been lost from Earth is a matter of debate3,8,9, but has tremendous implications for determining the bulk composition of Earth, its heat content and structure, and for constraining the modes and timescales of its geodynamical evolution3,7,9,10. Here, we show that compared to chondrites, Earth’s precursor bodies were enriched in Nd produced by the slow neutron capture process (s-process) of nucleosynthesis. This s-process excess leads to higher 142Nd/144Nd, and, after correction for this effect, the 142Nd/144Nd of chondrites and the accessible Earth are indistinguishable within 5 parts per million. The 142Nd offset between the accessible silicate Earth and chondrites, therefore, reflects a higher proportion of s-process Nd in the Earth, and not early differentiation processes. As such, our results obviate the need for hidden reservoir or super-chondritic Earth models, and imply a chondritic Sm/Nd for bulk Earth. Thus, although chondrites formed at greater heliocentric distance and contain a different mix of presolar components than Earth, they nevertheless are suitable proxies for Earth’s bulk chemical composition.

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